At present, star/delta connected power transformers used for three-phase electricity distribution networks have rated voltage levels in the range of 30 to 420 kV and rated currents up to 5000 A. Usually the voltage levels of the tap changers are .+-.5-10% of total rated transformer voltage, i.e. 22 kV or more, and the current ratings range from 300 to 3000 A.
Tap changing is the main method of providing regulation and control of the output voltage from each phase of a power transformer in an electrical distribution system. By connecting and disconnecting groups of winding turns, the power transformer voltage can be controlled despite a varying incoming voltage. The tap changer comprises a pair of contacts for connecting a point on the power transformer output winding into the circuit. The contacts are mechanically driven in an insulating oil bath.
Conventional tap changers can be divided into two categories, i.e. off-circuit tap changers and on-load tap changers. Off-circuit tap changers are those by which changes are made when the load current is off, while on-load tap changers are those in which the changes are carried out without interrupting load current. In order to control large high-voltage distribution networks and to maintain correct system voltages on industrial and domestic supplies, it is now common practice to use on-load tap changers. These have two main features: they have impedance in the form of either resistance or reactance to limit the current circulating between two taps; and a duplicate circuit is provided so that the load current can be carried by one circuit while switching is being effected in the other.
FIG. 1 shows an early form of reactor tap changer. There is only a single winding on the transformer. A current breaking switch is connected to each tap. Alternate switches are connected together to form two separate groups which are connected to the outer terminals of a separate mid-point reactor. The operating principle can be described as follows. At a first position, switch 1 is closed and the circuit is completed through half the reactor winding. To change taps by one position, switch 2 is closed in addition to switch 1. The reactor then bridges a winding section between two taps and gives a mid-voltage. To complete the tap change switch 1 is opened so that the circuit includes the second tap on the transformer winding. Tap changes can thus be effected by stepping tap by tap along the winding, executing the switch closing sequence each time.
Because a relatively large number of high current breaking switches are needed and consequently large dimensions and oil quantities are involved, this simple design was replaced by a new form in which two off-load tapping selectors and two current breaking or diverter switches were used. The selector and diverter switches are interlocked by mechanical gearing so that when either of the two tap selectors is to be moved, the corresponding diverter switch is open while the other switch is closed. After the process of the off-load tap selecting is finished, the state of the two diverter switches are changed, i.e. from on to off and from off to on.
There are also resistor-type tap changer arrangements in use. FIG. 2 shows a typical example comprising a tap selector and a diverter switch both of which are immersed in transformer oil. The tap selector selects the tappings, its electrical contacts being designed to carry but not to make or break load current. However, the diverter switch is designed to carry, make and break the load current. The transition resistors in the diverter circuit are used to perform two functions. Firstly, they bridge the tap in use and the tap to be used next for the purpose of transferring load current during tap changing. Secondly, they limit the circulating current due to the voltage difference between the two taps. As the arcuate contact moves in the direction of the arrow the load is first shared by the connected tap selectors on opposite sides and then transferred from one to the other when the contact comes to rest on the opposite terminals shown in the drawing.
The resistor-type tap changer is now used extensively by British and European electricity utility companies. This is because, relative to the reactor-type of tap changer, the modern resistor type of tap changer has a relatively high speed (due to the incorporation of energy storage springs in the driving mechanism); the intertap circulating current is of unity power factor; arcing time is short; contact life is extended (typically ten times longer) relative to the reactor-type; and maintenance requirements are reduced due to a lower rate of contamination of the transformer oil.
Although resistor-type tap changers have many advantages over the reactor type they are still mechanical. A main disadvantage is that the resistors cannot be continuously rated if their physical size is to be kept manageably small. Tap changing is still accompanied by arcing at the contacts, and transformer oil is still contaminated and should be replaced regularly. The arrangement of contacts makes the working life shorter and reliability lower. The mechanical drives have complicated gearing and shafting, the failure of which could be disastrous. Tap changers have a reputation for sticking contacts. While the speed of the diverter switch is in the range 50 to 100 ms, the selector switch is much slower with a speed of change time in the order of minutes.
There are several different schemes for solid state switch assisted on-load tap changers. The main intention of these schemes is to suppress the arc by incorporating thyristors in the diverter switches. For example, it has been proposed to superimpose thyristors across the arcing contacts of the diverter switches of the arrangement in FIG. 2. The arcing across the contacts is minimised because the making and breaking contacts will be shorted by the corresponding thyristors. Thus, while they complement existing on-load resistor-type tap changers, the tap changer itself is still mechanical in nature. The response speed is slow.
Thyristors usable in tap changers would be required to survive faults that may occur in the power system external to the transformer. For a large transformer having, say, a 240 MVA rating, the tap changer must be capable of passing 10 KA for a period of three seconds with a D.C. component superposed. The initial peak value of current in this case is about 25 kA, superposed. Normally the surge current rating given for a thyristor is for a 10 ms period only. The thyristor's surge current capability decreases with the increase of surge period. For a fault duration of three seconds, continuous current ratings would be applicable. Therefore, the full current level is the governing factor in determining the maximum permissible steady state current rating of the thyristors to be used in an electronic tap changer. Thus, at the present current rating level of commercially available thyristors (approximately 4000 A rms), devices must be connected in parallel to spread the load. Secondly, as a consequence the circuit design is complicated by the need to ensure parallel thyristors share current equally. Furthermore, power losses and therefore operating costs are high. Thus, thyristors are impracticable as power switches.